1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically to a silicon semiconductor device.
2. Description of Related Art
In conventional semiconductor devices, an impurity diffused region has been formed without being divided, regardless of the size of the impurity diffused region. However, in the case of a silicon substrate, if phosphorus, boron or antimony is diffused at a high degree of concentration, the lattice constant of the impurity diffused region has deviated from an inherent lattice constant of a silicon crystal. If a crystal strain caused by the deviation of the lattice constant exceeds an elastic limit of the silicon, crystal defect (misfit dislocation) occurs in the impurity diffused region and on the periphery of the impurity diffused region. This is well known to persons skilled in the art. For example, it was reported in Journal of Applied Physics, Vol. 38, pp81-87 (1967) that the misfit dislocation occurred in the case of diffusing P.sub.2 O.sub.5 at 1000.degree. C. for 15 minutes so as to obtain a phosphorus concentration of 3.times.10.sup.22 atoms/cm.sup.3 at a wafer surface. If this misfit dislocation penetrates an emitter-base junction and a base-collector junction of a bipolar transistor and a drain region and a source region of a MOS transistor, it becomes a cause for a junction leakage. Therefore, it has been a conventional practice to set the impurity diffusion concentration at a level which does not allow the misfit to occur.
As seen from the above, in the conventional semiconductor devices, even if it is desired to diffuse impurities at a high degree of concentration so as to reduce a resistance of the impurity diffused region, the impurity concentration has been restrained to prevent misfits which would otherwise occur in a surface of the diffused region because the impurity concentration becomes very high at the surface of the diffused region. As a result, it has not been possible to sufficiently reduce the resistance of the diffused region.